Electronic trip unit with user-adjustable sensitivity to current spikes

ABSTRACT

A circuit breaker trip unit with user-adjustable sensitivity to current spikes ( 22 ) includes a user-adjustable switch ( 30 ), a current sensor ( 32 ), an analog-to-digital (A/D) converter ( 34 ), a microprocessor ( 36 ), and a power supply ( 37 ). The current sensor ( 32 ) is electrically connected to the distribution circuit ( 10 ) and provides analog signals indicative of current measurements in the distribution circuit ( 10 ) to the A/D converter ( 34 ). The A/D converter ( 34 ) converts the analog signal to a digital line signal and presents the digital line signal, via a bus ( 38 ), to the microprocessor ( 36 ). The user-adjustable switch ( 30 ) is arranged to provide a signal indicative of a limit value, via a bus ( 40 ), to the microprocessor ( 36 ). The microprocessor ( 36 ) comprises a plurality of registers ( 42-48 ) and ROM ( 50 ) internal thereto. The ROM ( 50 ) includes trip unit application code, e.g., main functionality firmware, including initializing parameters, boot code, and a short circuit protection algorithm. The short circuit protection algorithm compares the line signal stored in the threshold register ( 48 ) to a predetermined threshold value stored in the ROM ( 50 ). If the line signal exceeds the threshold value, the microprocessor increments a peak count value stored in the peak count register ( 42 ). When the peak count reaches the limit value stored in the peak count register ( 42 ), a trip signal is generated by the microprocessor ( 36 ).

BACKGROUND OF THE INVENTION

The present invention relates generally to circuit breaker trip units.More specifically, the present invention relates to an electronic tripunit with adjustable sensitivity to current spikes.

The use of electronic trip units in electric circuit breakers is wellknown. Trip units can be used for, among other purposes, providing shortcircuit protection to an electrical distribution circuit. In thiscapacity, the trip unit samples current in the power lines of thedistribution system to detect a short circuit. If a short is detected,the trip unit provides a trip signal to an actuating device, such as atrip solenoid, within the circuit breaker. Upon receiving the tripsignal, the actuating device separates a pair of contacts within thecircuit breaker to open the distribution circuit and protect thedistribution circuit from damage caused by the short circuit.

The construction of an electronic trip unit is also known. Electronictrip units typically comprise voltage and/or current sensors, whichprovide analog signals indicative of the power line signals. The analogsignals are converted by an A/D (analog/digital) converter to digitalsignals, which are processed by a signal processor. Electronic tripunits further include RAM (random access memory), ROM (read only memory)and may also include EEPROM (electronic erasable programmable read onlymemory) all of which interface with the signal processor.

To detect short circuits in the distribution circuit, trip units monitorpeaks in the current within the power lines. Generally, trip unitscompare the current in the power lines to some threshold value. Forexample, this threshold value may be seven times the rated current ofthe circuit breaker. If the current in the power lines exceeds thisthreshold value, indicating a short circuit, the trip unit generates thetrip signal.

FIG. 1 shows a current waveform of fundamental frequency. In thewaveform shown, the current peak is higher than the threshold value and,therefore, this waveform is indicative of a short in the circuit. A tripunit would generate a trip signal if the waveform of FIG. 1 weredetected. FIG. 2, however, shows a current waveform with current spikescaused by high harmonic content or noise. Such current spikes can causethe circuit breaker to trip, even where no short circuit exists. Tripscaused by these current spikes can be a nuisance.

Attempts have been made to overcome this problem by using processingalgorithms to filter out the current spikes. While such is well suitedfor certain applications, such as drive systems, where current spikesare commonly generated, it is problematic in other applications, such ashigh-frequency systems (e.g., 400 Hz systems or resistive loadcircuits), where the user desires the trip unit to trip in response tosuch current spikes.

BRIEF SUMMARY OF THE INVENTION

In an exemplary embodiment of the invention, a circuit breaker trip unitwith user-adjustable sensitivity to current spikes comprises amicroprocessor arranged for receiving a line signal indicative ofcurrent within an electrical distribution circuit. The microprocessorcompares the line signal to a predetermined threshold value. If the linesignal exceeds the threshold value, the microprocessor increments a peakcount. A user-adjustable switch is arranged to provide themicroprocessor with a limit value. When the peak count reaches thislimit value, a trip signal is generated by the microprocessor.User-adjustable sensitivity to current spikes is beneficial, as itallows the user to tailor the sensitivity of the trip unit to aparticular application. For example, it allows the user to decreasesensitivity for applications, such as drive systems, where currentspikes are commonly generated. Conversely, the user can increasesensitivity for applications, such as high-frequency systems (e.g., 400Hz systems or resistive load circuits), where maximum sensitivity isdesired.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will now be described, by way of example only,with reference to the accompanying drawing in which:

FIG. 1 is a current waveform of fundamental frequency;

FIG. 2 is a current waveform with current spikes;

FIG. 3 is a schematic block diagram of a electric power distributioncircuit;

FIG. 4 is a schematic block diagram of a circuit breaker with anelectronic trip unit of the present invention;

FIG. 5 is a flow diagram of a short circuit protection program of thepresent invention;

FIG. 6 is a current waveform of fundamental frequency with a pluralityof samples for each half cycle;

FIG. 7 is a current waveform with current spikes and with a plurality ofsamples for each half cycle in accordance with the present invention;and

FIG. 8 is a flow diagram of an alternate method of short circuitprotection of the present invention.

DETAILED DESCRIPTION OF THE INVENTION

Referring to FIG. 3, an electrical power distribution circuit isgenerally shown at 10. Distribution circuit 10 comprises a source 12, anupstream circuit breaker 14, a downstream circuit breaker 16 and atleast one corresponding load 18. Any number of additional downstreamcircuit breakers 20 with corresponding loads 22 may be included. It willbe appreciated that breakers 14, 16, and 20 may be of similarconstruction.

Referring to FIG. 4, a general schematic of a circuit breaker isgenerally shown at 20. Circuit breaker 20 comprises a trip unit 23,actuating device 24, and contacts 26 all mounted within housing 28.Contacts 26 form part of distribution circuit 10 and are mechanicallyconnected to actuating device 24. Actuating device 24 is arranged toreceive a trip signal from trip unit 23, which is electrically connectedto distribution circuit 10. Upon receiving the trip signal, theactuating device 24 separates contacts 26 to stop the flow of current ina portion of the distribution circuit 10.

Trip unit 23 comprises a user-adjustable switch 30, a current sensor 32,an analog-to-digital (A/D) converter 34, a microprocessor 36, and apower supply 37. Power supply 37 is typically fed from the secondary ofcurrent sensor 32. Current sensor 32 is electrically connected todistribution circuit 10 by a line 33 and provides analog signalsindicative of current measurements in distribution circuit 10 to A/Dconverter 34, via a line 35. A/D converter 34 converts the analog signalto a digital line signal and presents the digital line signal, via bus38, to microprocessor 36. Power supply 37 is electrically connected todistribution circuit 10 by line 33 for providing operating power to A/Dconverter 34, switch 30, and microprocessor 36, via a line 41.

User-adjustable switch 30 is arranged to provide a signal indicative ofa limit value, via bus 40, to microprocessor 36. The user-adjustableswitch 30, for example, may be a binary coded decimal (BCD) encodedswitch that allows the user of the circuit breaker to alter the limitvalue provided to the microprocessor 36. Alternately, theuser-adjustable switch 30 may comprise a jumper bit or a user-selectableoption in non-volatile memory such as ROM (read only memory) 50.

Microprocessor 36 comprises a plurality of registers 42-48 and ROM 50internal thereto. ROM 50 includes trip unit application code, e.g., mainfunctionality firmware, including initializing parameters, boot code,and a short circuit protection algorithm. The plurality of registers42-48 comprises a register 48 for storing the line signal provided bythe A/D converter 34, a register 42 for storing the limit value providedby switch 30, and registers 44 and 46 for use by the microprocessor 36in executing the short circuit protection algorithm. It will beappreciated that RAM (random access memory), EEPROM (electronic erasableprogrammable read only memory) or any combination thereof may beemployed by the microprocessor 36 for memory purposes, as is well known.The EEPROM would include, e.g., operational parameters for theapplication code. It will also be appreciated that ROM 50 may beexternal to the microprocessor 36, as is well known. Further,communications within trip unit 22 can be provided through acommunications I/O port 51.

Referring to FIG. 5, the short circuit protection algorithm (program) isapplied to each of the phases of the power lines in distribution circuit10. The program is initiated preferably from the boot code at start-up,block 52, and proceeds immediately to block 54. At block 54 the programresets a sample count value stored in register 44 to zero. The programcontinues to block 56 where a peak count value stored in register 46 isreset to zero. At block 58, the program increments the sample countvalue in register 44. The program then waits a predetermined sampleperiod, block 60, and then proceeds to block 62 where a line signal inregister 48 is sampled. The sample period is a parameter stored in ROM50 and is equal to a fraction of the half-cycle of the current frequencyin the distribution circuit 10. For example, the sample period might beone-eighth of the half-cycle time. Thus, the line signal is sampledeight times per half-cycle (see, e.g., FIGS. 6 and 7).

At block 64, the program compares the line signal stored in register 48to a threshold value (e.g., seven times the rated current) stored in ROM50. If the line signal, which is indicative of the current level in thedistribution circuit 10, is less than the threshold value, the programcontinues to block 66. At block 66, the program compares the samplecount value in register 44 to a maximum sample value stored in ROM 50.The maximum sample value is equal to the number of samples perhalf-cycle of the current frequency in the distribution circuit. Usingthe example above, the maximum sample value would be eight. If thesample count value in register 44 is less than the maximum sample value,the program loops to block 58 where it increments the value in thesample count register 44 (to continue sampling the same half-cycle). Ifthe sample count is equal to the maximum, the program loops to block 54where it resets the sample count value in register 44 to zero (to begina new half-cycle).

Referring again to block 64, if the line signal stored in register 48 isgreater than the threshold value stored in ROM 50, the program continuesto block 68 where it increments the peak count value in register 46. Atblock 70, the program compares the peak count value in register 46 tothe peak limit value in register 42. If the peak count value is lessthan the peak limit value, the program continues to block 66 where, asdescribed above, the same half-cycle is sampled again or sampling of anew half-cycle begins. If the peak count value is equal to the peaklimit value, the program continues to block 72, where it initiates atrip signal. The program then ends at block 74.

FIGS. 6 and 7 show examples of a current signal sampled eight times perhalf-cycle. FIG. 6 represents a half-cycle with five line signals(samples) over the threshold value. In the short circuit detectionalgorithm of FIG. 5, if the peak limit value stored in register 42, asset by the user-adjustable switch 30, is five or less, the half-cycleshown in FIG. 6 would cause the breaker to trip. If set to six orhigher, the breaker would not trip. FIG. 7 represents a half-cycle withtwo line signals (samples) over the threshold value. In this case, ifthe user set the peak limit to three or greater, the breaker would nottrip. As shown in these examples, the user can adjust the sensitivity ofthe trip unit to current spikes by adjusting the switch 30.

Alternately, the short circuit protection algorithm (program) shown inFIG. 8 may be applied to each of the phases of the power lines indistribution circuit 10. The program is initiated preferably from theboot code at start-up, block 76, and proceeds immediately to block 78.At block 78, the program samples the line signal in register 48. Theprogram then continues to block 80 where it shifts the line signalstored in register 48 to register 46 and then continues to block 82. Atblock 82, the program waits a predetermined sample period, and thenproceeds to block 84 where a new line signal in register 48 is sampled.The sample period is a parameter stored in ROM 50 and is equal to afraction of the half-cycle of the current frequency in the distributioncircuit 10. For example, the sample period might be one-eighth of thehalf-cycle time, such that the line signal is sampled eight times perhalf-cycle.

At block 86, the program calculates the quantitative difference betweenthe previous line signal in register 46 and the current line signal inregister 48. The difference is compared to the limit value provided bythe user-adjustable switch 30 and stored in register 42. For example,the limit value may be equal to seven times the rated current. If thedifference is greater than the limit value, the program loops back toblock 80. If the difference is less than the limit value, the programcontinues to block 88 where the line signal in register 48 is comparedagainst a known threshold value (e.g., seven times the rated current)stored in ROM 50. If the line signal in register 48 is less than thethreshold value, the program loops back to block 80. If the line signalin register 48 is greater than the threshold value, the programcontinues to block 90, where it initiates a trip signal. The programthen ends at block 92.

The short circuit protection algorithm of FIG. 8 uses the rate of riseof two consecutive samples to detect current spikes. If the rate of riseis too steep (i.e., if the quantitative difference between the currentand previous line signals is greater than the limit value) thisindicates a current spike. The user can adjust the sensitivity of thetrip unit to current spikes by adjusting the limit value using switch30. If the user desires high sensitivity, the user can adjust switch 30to increase the limit value. Sensitivity can be reduced by decreasingthe limit value.

The short circuit protection algorithms of FIGS. 5 and 8 may furthercomprise a power-up feature that sets the trip unit for high sensitivityduring power-up and reduces the sensitivity during running state. Thisfeature, for example, can be used on the portions of distributionsystems that service electric drive motors. Alternately, switch 30 mayinclude a setting feature that would adjust the trip unit for use in a400 Hz application, where maximum sensitivity is needed.

The trip unit of the above-described invention allows the user of thecircuit breaker to adjust the trip unit's sensitivity to current spikes.This feature allows the user to decrease sensitivity for applicationssuch as drive systems, where current spikes are generated, and toincrease sensitivity for applications such as high-frequency systems,where maximum sensitivity is needed.

All of the aforementioned limits, settings or thresholds may be storedin any non-volatile memory or an EEPROM which can be altered bydownloading desired settings via communications I/O port 51. This wouldinclude remotely downloading such data when the unit is connected to asystem computer (not shown), either directly, over telephone lines, orany other suitable connection. It may also be preferred that such EEPROMcomprises a flash memory whereby such data is flashed, as is well known.

While the invention has been described with reference to a preferredembodiment, it will be understood by those skilled in the art thatvarious changes may be made and equivalents may be substituted forelements thereof without departing from the scope of the invention. Inaddition, many modifications may be made to adapt a particular situationor material to the teachings of the invention without departing from theessential scope thereof. Therefore, it is intended that the inventionnot be limited to the particular embodiment disclosed as the best modecontemplated for carrying out this invention, but that the inventionwill include all embodiments falling within the scope of the appendedclaims.

What is claimed is:
 1. A method of protection in an electronic tripunit, the method comprising the steps of: (a) selecting a limit value;(b) resetting a sample count; (c) resetting a peak count; (d)incrementing the sample count; (e) sensing an electrical signal toprovide a sensed signal indicative of an electrical characteristic ofthe electrical signal; (f) comparing said sensed signal to a threshold;(g) incrementing the peak count when said sensed signal exceeds saidthreshold; (h) if said sensed signal does not exceed said threshold,comparing the sample count to a maximum number of samples; (i) if thesample count is less than the maximum number of samples. returning tostep (d); (j) if the sample count is not less than the maximum number ofsamples, returning to step (b); (k) generating a trip signal when saidpeak count reaches said limit value; (l) if the peak count is less thanthe limit value, comparing the sample count to the maximum number ofsamples and returning to step (i).
 2. The method of claim 1 wherein saidselecting said limit value comprises selecting said limit value using aswitch.
 3. The method of claim 1 further comprising: sensing theelectrical signal a plurality of times each half-cycle of the electricalsignal.
 4. The method of claim 3 further comprising: resetting the peakcount each half-cycle of the electrical signal.
 5. The method of claim 1wherein said electrical characteristic comprise electrical current. 6.An electronic trip unit comprising: a switch for selecting a limitvalue; a sensor for sensing an electrical signal to provide a sensedsignal indicative of an electrical characteristic of the electricalsignal; and a signal processor responsive to said sensed signal, thesignal processor having a first register for storing the limit value, asecond register for storing a sample count, a third register for storinga peak count, and a fourth register for storing the sensed signal, thesignal processor further having memory for storing signals includingprogram signals defining an executable program for, comparing saidsensed signal to a threshold, incrementing the sample count at eachsensed signal, incrementing the peak count when said sensed signalexceeds said threshold, generating a trip signal when said peak countreaches said limit value, repeating the program if the sample count isless than a maximum number of samples, and resetting the sample countand the peak count if the sample count is not less than a maximum numberof samples.
 7. The electronic trip unit of claim 6 wherein said switchcomprises a binary coded decimal encoded switch.
 8. The electronic tripunit of claim 6 wherein said switch comprises a jumper bit option. 9.The electronic trip unit of claim 6, wherein said switch comprises anoption in a non-volatile memory.
 10. The electronic trip unit of claim 6wherein said program signals further define said executable program forsensing the electrical signal a plurality of times each half-cycle ofthe electrical signal.
 11. The electronic trip unit of claim 10 whereinsaid program signals further define said executable program forresetting the peak count each half-cycle of the electrical signal. 12.The electronic trip unit of claim 6 further comprising: a communicationport for communicating signals external of said electronic trip unit tosaid signal processor for remotely setting at least one of saidthreshold and said switch, whereby remotely setting said switch remotelyselects said limit value.
 13. A selective system including at least oneelectronic trip unit wherein said at least one electronic trip unitcomprise the electronic trip unit of claim
 6. 14. A method of protectionin an electronic trip unit, comprising: selecting a limit value; sensingan electrical signal to provide corresponding first and second sensedsignals, each indicative of an electrical characteristic of theelectrical signal; comparing said first and second sensed signals todetermine a rate of rise of said electrical characteristic; comparingsaid rate of rise to said limit value to detect a spike in saidelectrical characteristic; and generating a trip signal when said rateof rise is greater than said limit value.
 15. The method of claim 14wherein said selecting said limit value comprise selecting said limitvalue using a switch.
 16. The method of claim 14 wherein said electricalcharacteristic comprise electrical current.
 17. An electronic trip unitcomprising: a switch for selecting a limit value; a sensor for sensingan electrical signal to provide first and second sensed signals, eachindicative of an electrical characteristic of the electrical signal; anda signal processor responsive to said sensed signals, and having memoryfor storing signals including program signals defining an executableprogram for, comparing said first and second sensed signals to determinea rate of rise of said electrical characteristic, comparing said rate ofrise to said limit value to detect a spike in said electricalcharacteristic, and generating a trip signal when said rate of rise isgreater than said limit value.
 18. The electronic trip unit of claim 17wherein said switch comprises a binary coded decimal encoded switch. 19.The electronic trip unit of claim 17 wherein said switch comprises ajumper bit option.
 20. The electronic trip unit of claim 17 wherein saidswitch comprises an option in a non-volatile memory.
 21. The electronictrip unit of claim 17 wherein said electrical characteristic compriseelectrical current.
 22. The electronic trip unit of claim 17 furthercomprising: a communication port for communicating signals external ofsaid electronic trip unit to said signal processor for remotely settingat least one of said threshold and said switch, whereby remotely settingsaid switch remotely selected said limit value.
 23. A selective systemincluding at least one electronic trip unit wherein said at least oneelectronic trip unit comprise the electronic trip unit of claim
 17. 24.A method of protection in an electronic trip unit, comprising: selectinga limit value; sensing an electrical signal to provide a sensed signalindicative of an electrical characteristic of the electrical signal;comparing said sense signal to a threshold; incrementing a peak countwhen said sensed signal exceeds said threshold; and generating a tripsignal when said peak count reaches said limit value; wherein saidselecting said limit value further comprises setting said limit value toa first number of peak counts during an initial half-cycle after thedistribution circuit is powered up and then automatically increasingsaid limit value a second number of peak counts thereafter.
 25. Anelectronic trip unit comprising: a switch for selecting a limit value; asensor for sensing an electrical signal to provide a sensed signalindicative of an electrical characteristic of the electrical signal; anda signal processor responsive to said sensed signal, and having memoryfor storing signals including program signals defining an executableprogram for, setting said limit value to a first number of peak countsduring an initial half-cycle after the distribution circuit ispowered-up and then automatically increasing said limit value to asecond number of peak counts thereafter, comparing said sensed signal toa threshold, incrementing a peak count when said sensed signal exceedssaid threshold, and generating a trip signal when said peak countreaches said limit value.
 26. A method of protection in an electronictrip unit, comprising: selecting a limit value; sensing an electricalsignal to provide corresponding first and second sensed signals, eachindicative of an electrical characteristic of the electrical signal;comparing said first and second sensed signal to determine a rate ofrise of said electrical characteristic; comparing said rate of rise tosaid limit value to detect a spike in said electrical characteristic;and generating a trip signal when said rate of rise exceeds said limitvalue; wherein said selecting said limit value further comprises settingsaid limit value to a first rate of rise during an initial half-cycleafter the distribution circuit is powered-up and then automaticallyincreasing said limit value a second rate of rise thereafter.
 27. Anelectronic trip unit comprising: a switch for selecting a limit value; asensor for sensing an electrical signal to provide first and secondsensed signals, each indicative of an electrical characteristic of theelectrical signal; and a signal processor responsive to said sensedsignal, and having memory for storing signals including program signalsdefining an executable program for, comparing said first and secondsensed signal to determine a rate of rise of said electricalcharacteristic, comparing said rate of rise to said limit value todetect a spike in said electrical characteristic, and generating a tripsignal when said rate of rise exceeds said limit value, wherein settingsaid limit value comprises setting said limit value to a first rate ofrise during an initial half-cycle after the distribution circuit ispowered-up and then automatically increasing said limit value to asecond rate of rise thereafter.